CONVECTIVE FORECAST
VALID 06Z TUE 26/08 - 06Z WED 27/08 2003
ISSUED: 25/08 22:14Z
FORECASTER: DAHL

There is a moderate risk of severe thunderstorms forecast across S Italy and the Ionian Sea.

There is a slight risk of severe thunderstorms forecast across the central Mediterranean ... S Balkan and W Aegean Sea.

General thunderstorms are forecast across S ... NE and N Europe.

SYNOPSIS

Large and intense upper low is covering the NE parts of Europe ... yielding strong NWLY upper flow from E Greenland into eastern portions of central Europe. Vigorous embedded vort max ATTM over the N Atlantic ... is expected to cross Scandinavia tomorrow and reach the S North Sea/Denmark/Baltic Sea towards the end of the forecast period. Farther south ... Atlantic upper low is expected to deepen substantially during the period ... but should remain off-shore per latest model guidance. Weak southern-stream upper trough will continue its eastward way across the central Mediterranean tomorrow. At low levels ... N and E parts of the forecast area are dominated by deep polar air ... while moist/unstable subtropical air is residing over the S regions of the forecast area.

DISCUSSION

...Central Mediterranean...
Low-level airmass across the central and western Mediterranean has moistened substantially ... and 12 Z as well as 18 Z ascents reveal quite impressive thermodynamic profiles ... with MLCAPEs widely exceeding 2000 J/kg ... 18 Z Tripany (Sicily) ascent shows as much as 5000 J/kg MLCAPE.

850 hPa theta-e fields suggest that this airmass will cover the Tyrrhenian Sea ... Italy ... the Adriatic/Ionian Sea and the W Balkan States on Tuesday. Some variability in the depth of low-level moisture will likely exist ... also ... EML has not made it across all of the region as radiosonde data indicate. Generally, the shape of the profiles should improve towards the south. Also ... past days have shown continuous increase of the depth of low-level moisture across the central Mediterranean and there appears to be no reason to assume discontinuance of this trend. However ... current convective activity will likely provide extensive convective debris across Italy tomorrow ... PRIND that decaying elevated MCS will be present at the beginning of the period over the southern portions of Italy and the W Ionic Sea ... with extensive mid/upper level clouds covering much of SLGT area. Given sustained DCVA along with orographic forcing over Italy ... as well as numerous outflow boundaries from Monday night's convection ... a few storms may initiate rather early across Italy. Scenario remains somewhat uncertain since vigor of this activity will strongly depend on how how well the airmass recovers.

As short-wave trough crosses the Ionian Sea during the afternoon ... storms should either initiate along weak baroclinic zone over the Ionian Sea ... or they may initiate over S Italy and move into the Ionian. Vort max is expected to be accompanied by somewhat enhanced mid-level winds ... yielding 0-6 km shear on the order of 40 knots. If thermodynamic profiles will be as favorable as on Monday ... potential for a few significant severe weather events exists. Indications are that best kinematic and thermodynamic setup will exist S and E of the vort max within tongue of high theta-e over S Italy and the Ionian Sea. Reasonable chances exist that CAPEs will be well above 3000 J/kg ... coupled with about 30 to 40 knots deep-layer shear ... and development of severe multicellular storms and isolated supercells is anticipated. Large hail ... damaging winds and flash flooding will be possible with these storms. If mesocyclones indeed form ... giant hail will be possible ... low-level wind field apepars to be rather meager for a substantial tornado threat but low LCL heights over the water may support strong storm-generated low-level rotation to support a small tornado threat. Though several uncertainties are involved with this scenario ... a MDT risk is issued where shear and CAPE are expected to be maximized.

...N Europe...
Scattered shallow TSTMS will likely form again with diurnal heating in uncapped polar air over NE Europe.

Very strong shear will accompany vigorous vort max which will cross Scancinavia and the N Sea on Tuesday. Small hail and very gusty winds will likely occur with TSTMs that form in DCVA-regime ahead of this vort max. A few low-topped supercells may also occur ... and a brief severe hail and damaging wind event may occur. Also ... a brief tornado or two cannot be excluded. Expected areal coverage of the severe-TSTM events does not warrant a SLGT ATTM though.

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